A normal image is typically captured with a camera having a 30-70 degree field of view. Special cameras and lenses can produce much wider fields of view, and by using software multiple images can be stitched together into even wider fields of view all the way to a complete sphere. In addition, such images can be generated using a virtual camera viewing a virtual 3D world. These wide angle images often are high resolution and are created with a high dynamic range (HDR).
When viewing the foregoing wide angle images one often wants to be able to pan and zoom to examine a smaller portion of the image to appreciate the details. However, in order to display a sub-image of a wide angle, HDR image on a typical display device, tone mapping is needed. Tone mapping maps the tonal values of the HDR image to the limited dynamic range available on the display device. Typically, the whole image is “tone mapped” in advance of display so that much of the scene appears as natural as possible. This requires tradeoffs between very bright and very dark regions and maintaining a global contrast.
Unfortunately, this global tone mapping creates problems when zooming in on a wide angle image. For example, the details in a bright or dark region may not be clear owing to the use of the global contrast in tone mapping the pixels of the region. If it were known in advance what part of the image was to be displayed, then a very different tone mapping might be done—one which creates brightness and contrast level that make the details of the region stand out. A “local” tone map could be generated for the region of interest. However, it is impractical to pre-compute a local tone map for every possible sub-image that might be displayed.